This invention relates to heated humidifiers used in respiratory therapy. More particularly, it relates to a method of reducing the volume in the reservoirs of existing humidifiers and an apparatus for achieving such volume reduction cheaply and efficiently and for expanding the use of the humidifier to a larger universe of patients.
The use of heated humidifiers in respiratory therapy is a widespread and commonly employed practice in hospitals. Given the necessity for imposing stringent cost controls on spiraling hospital expenses, cost-cutting improvements attending the use of such humidifiers is quite desirable. The problems exhibited by prior art humidifiers, and the present needs to be addressed, are diverse. At the outset it is essential to recognize that any improvement should be compatible with existing equipment. Since the impetus for improvement is lower cost in patient care, any improvement requiring the replacement of old equipment, or expensive modification of existing apparatus, is an illusory benefit at best, and is self-defeating at worst. The method herein provides a simultaneous solution to the variety of problems associated with several designs of heated humidifiers used in respiratory therapy and to the needs of hospitals in achieving a significant cost reduction attending their use.
Heated humidifiers are used to add sterile water vapor to gas being inhaled by patients whose upper airway has been bypassed or rendered ineffective in preventing desiccation of the respiratory mucosa. Such patients typically have an endotracheal or tracheotomy tube in their trachea so that the normal physiological means (the nose, mouth, trachea, etc.) of heating and humidifying inhaled gas has been bypassed. The heated humidifier warms and humidifies the gas by causing it to bubble through a volume of heated sterile water. The function of the heated humidifier is to heat and humidify the gas to an appropriate, constant temperature and saturation level so that upon entry into the tracheal tube its temperature is between 85.degree. and 98.6.degree. F. and it is saturated with sterile water vapor. To date, this has been done by using humidifiers with a relatively large water reservoir, since a large volume aids in minimizing temperature fluctuation once the operational temperature has been achieved and extends the time between refills. However, since the reservoir of the heated humidifiers and the sterile water contained therein requires change at least every 24 hours to insure maintenance of sterile conditions, a large volume reservoir leads to appreciable waste by incomplete utilization of the sterile water. Such waste is an additional cost factor of patient care. For example, one heated humidifier commonly used has a reservoir whose volume from " empty" to "refill" is 475 ml, with a volume from "refill" to "full" of 300 ml. Therefore the amount of sterile water discarded per day may vary between 475 and 775 ml, which equates to a daily waste between $1.50-2.50 per humidifier.
Another undesirable characteristic of heated humidifiers with a large reservoir is the relatively high compressible volume, or compliance volume, attending their size. By compliance volume is meant that volume of gas used for development of pressure in the humidifier; it is an amount which plays no role in the ventilation of the patient, but is an amount of gas needed to be supplied by the ventilator. The total gas supplied by the ventilator is the sum of the volume supplied to the patient plus the compliance volume of the system (ventilator, humidifier, and breathing circuit). The compliance volume itself varies with the water level in the reservoir, and unless the reservoir is supplied with a constant level control the compliance volume varies during use. This leads to a varying gas volume delivered to the patient since the total gas output from the ventilator typically is constant at a given setting. Using the heated humidifier previously exemplified, the compliance factor with the water level at "full" is 1.0 ml per cm of water pressure. At a peak inspiratory pressure of 40 cm of water 40 ml must be added to the ventilator output merely because of humidifier compliance. When the water level is at "refill" the compliance factor is 1.3 ml per cm of water pressure, leading to an additional 52 ml ventilator gas output needed to deliver proper gas volume to the patient.
Since the compliance volume represents an amount of gas not utilized by the patient, it also represents wasted gas, an additional cost for which no benefit is received. Because wasted gas nevertheless is heated and humidified, which requires the expenditure of energy, the compliance volume also represents an energy cost for which no benefit is received.
The compliance volume increases with an increase in peak inspiratory pressure. As the pressure necessary to ventilate the patient increases, the significance attached to the compliance volume increases because the latter robs a proportionately larger share of gas of the ventilator output. This becomes especially significant in pediatric and neonatal patients, whose inelastic lungs require a peak inspiratory pressure about twice that for adult patients, but whose gas volume need is perhaps only 5% to 25% that of the adult. In such cases, a relatively large fraction--perhaps from about 1/4 to about 1/2--of the ventilator output is not utilized, thereby leading to gross waste. The compressible volume is directly related to the internal volumes of the system components; a higher compressible volume means that the internal volume is higher. In turn, that means that a larger volume must be inhaled by the patient to create the negative pressure needed to trigger the inspiratory cycle of the ventilator. Neonatal patients, as a rule, cannot inhale sufficient volume to trigger the ventilator when an "adult-size" humidifier is used. As a result manufacturers have developed pediatric-size humidifiers, which means that hospitals with both pediatric and adult patients need to acquire and maintain two sizes of humidifiers, further adding to patient cost.
The large reservoir also requires a relatively high wattage heater, both to bring the larger volume of water to temperature in a reasonable time and to maintain it once there. The result is a relatively high degree of energy waste. A desirable improvement would be to supply only enough heat as is necessary to vaporize that amount of water required to achieve the temperature and humidity desired.
The marketplace needs a heated humidifier with less waste and greater utility. It needs a unit with less energy loss, less waste of humidifying water, lower compliance volume, and reduced labor costs attending the maintenance of reservoir water level; it needs a unit usable with both neonatal and adult patients and it needs to achieve all these ends without requiring replacement of existing units by expensive new equipment.
The invention herein is remarkable not only in its simplicity but also in its achieving the varied needs of the user efficiently and at low cost. The invention herein is a method of maintaining in the reservoir of large volume humidifiers a relatively small, and in one embodiment a constant, volume of water so as to use little energy above that necessary to vaporize the required amount. Because the water volume is small there is little waste when the reservoir is changed. Because the resulting compressible volume of the reservoir is small the compliance volume is reduced substantially, so that the same humidifier is usable both for pediatric and neonatal patients as well as adults. Finally, the embodiment incorporating constant water level control eliminates the labor costs and ventilation interruptions caused by periodic refilling.